1254 FLIGHT International, 29 October (977
2,094m in February this year, a straight-line record that
remains unbeaten.
Surprisingly few attempts have emanated from the US,
and none appeared likely to set any records until this
year. The RAeS man-powered aircraft group recorded in
its report this year "a flight of 50-60ft, at Quonset Point,
Rhode Island, on April 21" by a machine called Olympian.
This was the first American man-powered aircraft flight
reported to the Society. It terminated abruptly when the
wing failed at the root. Although it had not been reported
Gossamer Condor (at that time called Gossamer Gull) had
already made its first hops in California and was being
prepared to take the big Kramer Prize.
Looking back on all the pre-Gossamer Condor attempts,
some similarities are apparent. Most aircraft had wings
with an aspect ratio of 20 or more, and wing loading was
pegged between 0-7 and lib/ft2. The high-aspect-ratio
wings were generally inspired by gliders, which are also
designed for low-drag flight at low forward speeds. In most
cases a wing section which operated at a relatively high
lift coefficient was used, mainly to minimise the wing area,
and cruising speeds were usually around 15-20 m.p.h.
Power needed for straight and level flight had to be in the
region of 0-35-0-4 h.p. This includes an allowance for
"ground effect"—aircraft flying close to the ground experi
ence a reduced induced-drag contribution to total drag—
but more power is needed when turning. Many designs,
because of imperfections in the streamlining and some
unexpected low-airspeed effects, often needed more power
than predicted. In almost every case the lightweight
structure was complex, especially when designers tried to
build unbraced wings. Not only did the thousands of
individual components take many man-hours to produce,
but the slightest accident would cause time-consuming
damage. Every project tended to be rebuilt with modifica
tions after the most minor scrape.
Gossamer Condor was born not from thoughts of a man-
powered glider but from the concept of a man-powered
hang glider. Designer Paul MacCready, the man who will
receive the £50,000 cheque, hit on this idea last year. After
a few trial calculations he became convinced that he could
win the Kremer Prize, but admits to wondering if he would
be pipped at the post. The Japanese team in fact came
close to robbing him of success during a trial flight early
this year, when the Stork flew about three-quarters of the
Kremer course before a wingtip touched the ground.
MacCready is well known in gliding circles. He was world
champion in 1956, and his own invention, the MacCready
speed-to-fly ring, is a device used by a large number of
gliding pilots. In 1970 he set up his own company, Aero-
Vironment, to specialise in industrial aerodynamics. Among
the men he works with is Peter Lissaman, an aero-
dynamicist who set up computer programs to produce
aerofoil and propeller-blade sections optimised for
Gossamer Condor. MacCready chose the name because it is
light and flimsy, and to relate with the Californian Condor,
the largest and slowest-flying soaring bird in North
America. The bird is an endangered species, "there are
about as many Californian Condors in existence as there
are human-powered aircraft," says MacCready, recalling
that there are only 30 or so birds nesting close to Shafter
airfield, where the record-breaking flight was made.
The aircraft has a large, swept mainplane with a plan-
form like that of a hang glider but of much larger span.
For control a canard surface is placed ahead and below
the mainplane, with nothing more than a 2in-diameter
bowsprit playing the part of fuselage. The pilot sits in the
fin, which hangs beneath the mainplane centre-section.
There is no wetted area above that required for stable and
controllable flight but, due to the very large mainplane,
airframe total area is much more than that of any other
man-powered aircraft design.
MacOready reasoned that a lower-aspect-ratio wing,
externally braced and optimised to fly at a lower airspeed
than a "conventional" man-powered aircraft, would produce
the best minimum-power design. Gossamer Condor has
about 70 external bracing wires radiating from strategic
points to attachments on the wing and fuselage bowsprit.
Stainless-steel piano wire ranging from 0 • 022in to 0 • 036in
diameter is used. Cruising speed is only 11 m.p.h.—the
aircraft takes half as long again as its competitors to
complete the Kremer course:—but the cruising power is
only about 0-35 h.p. It was this efficiency that clinched
the prize.
A crucial design objective was low structural weight.
The lack of excess wetted area and use of external bracing
helped a lot, but building a wing as big as an F.27's and
achieving a total airframe weight of only 701b was not an
easy task. MacCready used balsa, corrugated cardboard and
Styrofoam with thin Mylar covering, and wing and
stabiliser spars made of 2in-diameter tubing with thick
nesses varying from 0-013 to 0-022in. The wing has been
damaged and repaired in three places and could be as
much as 81b overweight says MacCready. The design was
stressed to an operating load factor of 1 • 25g with a 1401b
pilot. If a component broke it was replaced with extra
material until adequate strength was achieved. If large
components appeared to be over-strong they were
weakened, including thin styrofoam ribs in the wing leading
edge. Ease of repair has been a major factor contributing
to the aircraft's rapid development.
Gossamer Condor's control system is conventional but
has a couple of novelties in the lateral system. Wings-level
attitude is maintained by tab deflections on the flexible
stabiliser trailing-edge, and although sloppiness causes the
whole surface to cant at an angle when tab deflections are
applied, MacCready is not worried. Once cruising, this is
adequate to keep the aircraft on a straight line; but to
turn requires a more effective control, so the pilot warps
alu-
wall
KEY
2in-diameter heat-treated
minium tube 0-OI5in-0-022in
thickness
2 Mylar film (0005mm top skin,
0-0025mm bottom skin)
3 Chemically milled aluminium tube,
0-25in diameter
4 Balsawood
5 Corrugated-cardboard leading-edge
skin
6 Mylar tape
7 Piano-wire bracing, 0-022in-0-036in
diameter
8 Cord adjusting loops at wire ends
9 Securing tape
10 Styrofoam
11 Control stick: up/down for pitch,
twist for limited roll/yaw
12 Control wires
13 Pitch-control wires
14 Control tabs
15 Wing-warp wires
16 Ventilation
17 Air vent
18 Window
19 ASI generator
20 Instruments
21 I2ft-diameter
r.p.m.)
airscrew (115-120
MAN-POWERED AIRCRAFT: LEADING DATA
Wing Wing
Span area Aspect Empty wt Flying wt loading
(ft) (ft2) ratio (lb) (lb) (lb/ft*)
Sumpac 80 0 300 21-3 128 269 0-90
Puffih II 930 390 220 140 290 0-78
Mercury 1200 485 300 178 329 0-68
Jupiter 80 0 300 20-7 146 297 0-99
Toucan I 1230 600 250 209 519 0-85
Malliga 640 262 15-6 113 239 0-91
Linnet IV 830 316 . 21-9 119 237 0-75
Stork 68-8 234 20-3 79 207 0-88
Gossamer
Condor 960 720* 12-8* 70 210 0-29*
* Mainplane only. Canard area is approximately 96fU, span 23ft.